In plasma arc cutting machines for example, which are a kind of plasma processing apparatus, cutting is done by supplying a high-temperature, high-velocity plasma jet from a plasma torch to a workpiece (an object material) to melt a localized area of the workpiece and blowing the molten portion off. During the cutting operation, a pilot arc is generated between the electrode and the nozzle within the plasma torch while feeding plasma gas to the plasma torch, and then a main arc is established between the electrode and the workpiece, initiated by the pilot arc, thereby forming a high-temperature plasma arc. This plasma arc is narrowed down by the nozzle of the plasma torch, so that the plasma jet (a high temperature, high-velocity jet stream) suited for the cutting operation can be obtained.
Owing to the influence of the high-temperature plasma and arc discharging, the electrode and nozzle within the plasma torch are consumed each time cutting is done and arc generation occurs, and finally, the generation of the plasma arc is hindered and the cutting quality deteriorates. Therefore, replacement in proper timing is necessary. The electrode and nozzle (hereinafter referred to as “consumable part”) are able to serve several hundreds of cutting operations and their duration is several hours (cutting time). They require replacement about once or twice during 24-hour operation and it is still the case that the timing for the replacement is dependent on the operator's experimental judgment.
It sometimes happens due to overlooking caused by the operator's lack of experience or negligence that the consumable part is still used even though its lifetime has already expired. This leads to such a problem as cutting defects or damage to the torch. On the other hand, if the consumable part is replaced too early in advance of the expiration of the lifetime, cutting defects and damage to the torch can be avoided but the expensive consumable part will be wasted, resulting in increased running cost. To solve this, there have been previously proposed various techniques associated with lifetime management with a view to effective use of the consumable part.
For example, Japanese Patent No. 2611337 has proposed a technique associated with a lifetime judgment circuit in which the consumption of the consumable part is estimated by processing and accumulating detected data on the number of arcing events, arcing time and arc current by use of specified arithmetic expressions, and based on the result of a comparison between an estimated consumption value and an allowable consumption value, warning, suspension of the operation, or displaying of a remaining service time ratio is effected. Japanese Patent Kokai Publication No. 5-245645 discloses a technical concept that is basically the same as that of Japanese Patent No. 2611337 in that expended processing time is accumulated to be compared to an output from a setting circuit and that parameters are selected with processing current set as a variable. In addition, Japanese Patent Kokai Publication No. 9-216066 discloses a technique in which the lifetime of the consumable part is determined by measuring the temperature of cooling water which rises as the electrode is consumed.
In a typical plasma arc cutting machine, the value of arc current is changed according to the thickness of a workpiece to be cut, and the nozzle and the electrode are replaced according to the value of arc current. The reason for replacing the nozzle and the electrode in accordance with the value of arc current is that there are an optimum nozzle and electrode for each arc current value, which have good lifetimes (durability) and provide good cutting quality. The consumable part is sometimes replaced before expiration of its lifetime in other occasions than the case described above. These occasions are when highly-accurate cutting quality is required and when the lifetime of the consumable part is likely to expire in the course of cutting operation. To cope with such situations, a plurality of consumable parts are selectively properly used, when performing cutting operation with a plasma arc cutting machine.
The technique disclosed in Japanese Patent No. 2611337, however, is designed to include a lifetime judgment circuit for a single consumable part. Therefore, when applied to a case where a plurality of consumable parts are selectively used as described earlier, it cannot perform data management nor arithmetic operations on every consumable part, proving to be unsuccessful in accurate lifetime assessments of a plurality of consumable parts. The techniques disclosed in Japanese Patent Kokai Publications No. 5-245645 and No. 9-216066 are also intended to make an accurate lifetime assessment for a single consumable part only and therefore suffer from the same problem as that of Japanese Patent No. 2611337.
The present invention is directed to overcoming the foregoing shortcomings and a primary object of the invention is therefore to provide a plasma processing apparatus capable of making an accurate lifetime assessment for each of plural consumable parts so that effective use of the consumable parts becomes possible contributing to a saving in the running cost.